When skeletal muscle is stimulated to contract rhythmically, there is a rapid decrease in vascular resistance (active hyperemia). Adenosine, a potent vasodilator, is known to accumulate in contracting skeletal muscle and has been suggested as a mediator of active hyperemia. The major aims of the proposed study are (1) to see if changes in tissue concentrations of adenosine correlate in time with changes in dog gracilis muscle vascular resistance during rhythmical contractions; (2) to characterize in vitro the efficacy of dipyridamole and several different adenosine deaminase inhibitors on whole blood samples from dogs; (3) to determine if intra-arterial infusions of dipyridamole and adenosine deaminase inhibitors alter tissue oxygen consumption, adenine nucleotide pools, adenosine concentration, and lactate concentration in resting and contracting skeletal muscle; and (4) to determine if these compounds enhance the accumulation of adenosine in contracting muscle and potentiate active hyperemia. Tissue biopsies will be taken before, during, and after 20 min. periods of electrical stimulation. Tissue and blood samples will be analyzed for nucleotides, nucleosides, and glycolytic intermediates by high pressure liquid chromatography or enzymatic assay.